U.S. Pat. No. 5,725,508 discloses a medication delivery pen having a reusable pen body assembly and a disposable cartridge assembly that are threadedly engageable with one another. A portion of the pen body assembly projects into the cartridge holder assembly for driving a cartridge plunger distances that are selected in accordance with a desired dose of medication to be delivered. The pen body assembly includes a generally cylindrical driver being slideably inserted in the housing, and a dose knob which is a hollow generally cylindrical structure. The dose knob is spline mounted over the driver within the housing of the pen body assembly, and relative rotation between the driver and the dose knob is thereby prevented.
A desired dose is set by rotating the dose knob, and during injection the dose knob also rotates. In order to prevent the piston rod from rotating during dose setting, but allowing it to rotate during injection, the pen body assembly comprises a clutch arrangement. The clutch arrangement constitutes additional mechanical parts of the medication delivery pen, and it renders the pen relatively complex to manufacture and operate.
Other similar injection devices are disclosed in WO 2006/058883 and WO 2006/084876, both disclosing injection devices in which a clutch arrangement being switchable between a dose setting mode and an injection mode is necessary in order to operate the injection properly.
US 2007/0088288 discloses a fixed dose injection pen. To execute a dose setting and ejection cycle using this pen a user has to perform four operations on the pen. First the dosage button must be rotated to a zero dose radial position relative to the housing. Thereafter the dosage button is pulled proximally out of the housing. Then the dosage button is rotated to a stop in order to set the dose, and finally the dosage button is pushed distally to eject the dose. This four-step dose setting and ejection procedure may be cumbersome to some users.